Garmin app for older android OS?

You should use Garmin Connect. Try sideloading it from some other site. Make sure you use a reliable source.

Another alternative is to upgrade the Note 3 to Android 9: https://www.youtube.com/watch?v=1k9pahEKC4Q 

Thanks for the reply... but i wouldn't know where to start with a reliable source these days. Can you recommend one?

Unless I can install it onto my PC and install it on the phone from there?

I'm quite Pc savvy but a bit clueless when it comes to smart phones :)

I would probably try this one: https://www.apkmirror.com/apk/garmin/garmin-connect-mobile/garmin-connect-mobile-4-29-release/garmin-connect-4-29-android-apk-download/

You will have to run an old version since from 4.30 Android 6.0 is required.

Thank you... that's looking very promising :)

EDIT... tried to install v4.29 as that was the one for Android 5.0 but it wouldn't install... back to the drawing board :(

After going into the security settings to enable 3rd party installations, i tried again to no avail... but I ended up trying the even earlier version, 4.22 which does seem to have installed without a hitch.

Thanks for your help e7andy. I wouldn't have known where to start without you. Much appreciated. 

Great! There are probably some functions that will not work as intended since it is an old version.